This invention relates to an apparatus for inspecting labels applied to containers as they move on a conveyor line. More specifically the inspection given by the apparatus of this invention simulates a casual visual inspection that an average person might give the container. The purpose of this invention is not to detect every minute flaw in the label, but rather those flaws that the average person would detect with a casual visual inspection. The apparatus further includes means for rejecting or ejecting from the conveyor line those containers having labels found defective either as the result of label damage or as the result of errors in the application or positioning of the label on the container.
The label inspection apparatus of the present invention is particularly suited to inspect labels on bottles, such as for example blow molded plastic bottles, that are generally rectangular in horizontal cross-section in the label area and where the label is applied to a generally flat or slightly convex as opposed to a round side surface of the bottle. The inspection takes place automatically as the containers are moved by a conveyor without interruption of the conveyor line.
One practical adaptation of the present invention is for use in inspecting labels on one quart plastic motor oil bottles that normally have a height of six to eleven inches, a length of three to five inches, and a thickness of two to three and a half inches in the label area. These containers are rectangular in cross-section with slightly convex side panels to which the labels to be inspected are affixed. There may be a label on each of the two side panels. These labels may be affixed by adhesive or by heat transfer or other means. The labels are of various sizes and shapes usually related to the panel area available for their application. Portions of the container may appear in areas within the label outline.
Automatic label inspection devices are known in the art. For example, U.S. Pat. No. 4,270,863 discloses such a device. This patent describes a device for sorting good and bad bottles by comparing a camera's or diode array's view of the label against minimum and maximum values stored in memory arrays. These minimum and maximum values are obtained from a plurality of reference bottles which are placed before the sensor array by the operator and read by the device. The minimum and maximum light values received from each element of the sensor array from the reference bottles are stored in the memory arrays. The patent discloses various techniques for "scoring" the defects as determined by the minimum and maximum values. This device has disadvantages. It requires a number of manual operations and decisions by the operator including the placement of the reference bottles before the sensor arrays. In contrast, the device of the present invention includes just one operator control, a push button. The patent reference uses the minimum and maximum values as the limits for accepting or rejecting containers. The device of the present invention samples a number of containers and uses a statistical analysis calculating the mean and standard deviation for each pixel or sensor reading. Limits are generated that statistically describe an entire population (production run) of readings based on the sample population. Other examples of label inspection devices are described in U.S. Pat. Nos. 3,553,041, 4,589,141, 4,311,914, and 4,244,650.
In addition to the advantages previously mentioned, the present invention also has the capability to inspect labels with the stretch-and-shrink problem generated by the heat transfer label applicators and also labels of metal foil or other highly reflective metalized paper surfaces. The hardware and software of the present invention, using statistical techniques, provide adequate inspection for such labels and determine the inspection technique that will work best for the particular type label.
The inspection label device of the present invention inspects labels on containers moving on a conveyor past the inspection station. The movement of the conveyor is monitored by an incremental encoder that outputs an electrical pulse to a microprocessor unit for each small increment of conveyor travel. As a container enters the apparatus, its presence is first detected by a photodetector connected to an amplifier/comparator on the microprocessor unit. The apparatus has lamps for illuminating the labels as the container moves through the apparatus. Sensing arrays comprising photodetectors detect the reflectivity values of pixels on each label. For example, as will be explained in connection with the preferred embodiment of the invention, there may be eight photodetectors in each array each of which measures the reflectivity value of eight areas or pixels across the label as the container moves past the array. With eight photodetectors in vertical alignment each measuring the reflectivity value of eight pixels, there are a total of 64 pixels for each label. The photodetectors are arranged and the increments of measurements are such that the 8.times.8 pixel matrix extends over substantially the entire label for complete label coverage and detection.
In brief, the inspection apparatus of this invention studies the reflectivity characteristics of a statistical sample population of containers and then judges the following containers by the values obtained and rules set forth in its program. It has an "inspection" mode and a "detection" mode, each of which includes a "learn cycle". In the learn cycle the device studies a sufficiently large statistical sample of containers to correctly represent the population of containers to be inspected. Each label of the container samples is read, i.e., the reflectivity values measured for each pixel of each label, and from this the apparatus computes the means and three standard deviations for each pixel or photodetector for the sample labels. The means are computed by taking the sums of pixel or average photodetector readings and dividing by the number of containers sampled. These values represent the average light reflectivity for each reading point or pixel in the case of the "inspection" mode, or the average light reflectivity for each photodetector in the case of the "detection" mode. The means values are stored in a table. The three standard deviations are also computed and these values are retained in another table. The three standard deviations value when added to the means value forms an upper limit for the reflectivity for that pixel or for that detector, and when substracted from the means value forms the lower limit for that pixel or for that detector. Statistically, these limits should contain 99.7% of the samples in a normal population (production run) of bottles.
Once these limits are established from the samples in the learn cycle, they are used to determine the acceptability of the containers that follow. As each container passes through the label inspection apparatus, each pixel or eye (detector) is scored. If all of the pixels or eye readings are within the limits determined by the means plus and minus the three standard deviations then the container is allowed to pass. If a pixel or eye is outside of the limits, it is scored with a number representing how far it is outside those limits. The manner of scoring may be selected depending on how strict or liberal the inspection is to be. For example, a label can be failed if only one pixel or eye is outside the limit, or it may require that an individual pixel or eye error exceed the limit by a certain amount or that there be a limit on the total errors per label, or a combination of these.
Hence it is a primary objective of this invention to provide a label inspection apparatus that is easy to operate, relatively inexpensive, and that reliably simulates a casual visual inspection that an average person might give the container. The invention further provides such an apparatus that is easily adaptable for use with highly reflective labels such as those made of metal foil or metalized paper.